1. Field of the Invention
The present invention relates to a method for transferring heat from one liquid to another, to heat exchangers for performing the method, and particularly to such exchangers wherein the coolant is passed through a network of conduction passages across which a cooling medium is passed.
2. Description of the Prior Art
Hitherto, water cooled heat exchangers operating at temperatures below 100.degree. C. at atmospheric pressure and in temperate ambient conditions suffer the shortcoming of poor effiency due to the low heat gradient between the coolant such as flows through an engine and the cooling medium. In the past this problem has been overcome by one of, or a combination of two methods.
In a first method the flow rate of the cooling medium across to the passages is increased. The problems associated with this solution are two-fold. Firstly, increased energy is required to pump the cooling medium across the passages and secondly, a greater amount of cooling medium is required. This can be a particular problem in closed cycle, fluid cooling medium systems.
In a second method the length of passage is increased, increasing the contact area between the coolant and the cooling medium. The problems associated with this method are increased cost of manufacture and increased required coolant pump capacity to overcome the frictional losses incurred in the increased passage length.
It is an object of the present invention to provide a method and apparatus able to avoid or at least ameliorate the above listed shortcomings by utilizing latent heat of vaporization to assist in the cooling process, the boiling point of the cooling medium being reduced below the temperature of the coolant by reducing the pressure of the cooling medium.